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Pulses of Deformation Reveal Frequently Recurring Shallow Magmatic Activity Beneath the Main Ethiopian Rift

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Pulses of Deformation Reveal Frequently Recurring Shallow Magmatic Activity Beneath the Main Ethiopian Rift Article Volume 12, Number 9 10 September 2011 Q0AB10, doi:10.1029/2011GC003662 ISSN: 1525‐2027 Pulses of deformation reveal frequently recurring shallow magmatic activity beneath the Main Ethiopian Rift J. Biggs and I. D. Bastow Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Street, Bristol BS8 1RJ, UK ([email protected]) D. Keir National Oceanography Centre, Southampton SO14 3ZH, UK E. Lewi IGSSA, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia [1] Magmatism strongly influences continental rift development, yet the mechanism, distribution, and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism, and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma‐rich geological record, only one eruption has been historically recorded, and no dedicated monitoring networks exist. Consequently, the present‐day magmatic processes in the region remain poorly documented, and the associated hazards are neglected. We use satellite‐ based interferometric synthetic aperture radar observations to demonstrate that significant deformation has occurring at four volcanic edifices in the MER (Alutu, Corbetti, Bora, and Haledebi) from 1993 to 2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10–15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (<10 km), frequently replenished zone of magma storage associated with volcanic edifices and add to the growing body of observations that indicate shallow magmatic processes operating on a decadal timescale are ubiquitous throughout the East African Rift. In the absence of detailed historical records of volcanic activity, satellite‐ based observations of monitoring parameters, such as deformation, could play an important role in assessing volcanic hazard. Components: 6400 words, 6 figures, 2 tables. Keywords: InSAR; continental rifting; magmatic processes; volcano deformation. Index Terms: 1240 Geodesy and Gravity: Satellite geodesy: results (6929, 7215, 7230, 7240); 8105 Tectonophysics: Continental margins: divergent (1212, 8124); 8485 Volcanology: Remote sensing of volcanoes (4337). Received 15 April 2011; Revised 15 June 2011; Accepted 15 June 2011; Published 10 September 2011. Biggs, J., I. D. Bastow, D. Keir, and E. Lewi (2011), Pulses of deformation reveal frequently recurring shallow magmatic activity beneath the Main Ethiopian Rift, Geochem. Geophys. Geosyst., 12, Q0AB10, doi:10.1029/2011GC003662. Theme: Magma-Rich Extensional Regimes Guest Editors: R. Meyer, J. van Wijk, A. Breivik, and C. Tegner Copyright 2011 by the American Geophysical Union 1 of 11 Geochemistry Geophysics 3 BIGGS ET AL.: DEFORMING VOLCANOES IN THE MER 10.1029/2011GC003662 Geosystems G 1. Introduction intrusion expected to play a lesser role in accom- modating extension [Ebinger and Hayward, 1996]. [2] As a continental rift develops towards a new [5] Despite the numerous studies concerning the oceanic spreading center, extension initially accom- behavior and significance of magmatism on geo- modated in a broad zone of faulting and ductile logical timescales, little is known regarding the stretching transitions towards a narrower zone of short‐term activity of these volcanoes. Eruptions are focused magmatic intrusion [e.g., Ebinger and believed to have occurred in the 1800s at Fentale and Casey, 2001]. The spatial and temporal variations Kone [Rampey et al., 2010] and many others are in strain partitioning between faulting and magma- quotes as having undated lava flows with youthful‐ tism during progressive extension plays a vital role looking morphology [Siebert and Simkin, 2002]. in determining the continental rheology as well as The World Bank report on volcanic hazard lists local volcanic and seismic hazard. all these volcanoes at a zero level of monitoring and [3] Recent geophysical and geochemical experi- the highest level of uncertainty in terms of hazard ments in the Main Ethiopian Rift (MER) highlight and risk [Aspinall et al., 2011]. the importance of magmatism in achieving exten- [6] Here we present interferometric synthetic aper- sion during continental breakup [e.g., Keranen et al., ture radar (InSAR) observations from the seismi- 2004; Mackenzie et al., 2005; Bastow et al., 2011]. cally active MER (Figure 1), which encompasses The 60 km wide rift valley is bound by steep, large the transition between mechanical and magmatic offset, border faults that accommodated extension extension during the breakup of a continent [e.g., during the Miocene [Woldegabriel et al., 1990; Ebinger and Casey, 2001; Wolfenden et al., 2004]. Wolfenden et al., 2004]. Since the Quaternary, strain We investigate the spatial and temporal character- has localized to discrete, ∼20 km wide zones of istics of deformation and explore its source location, volcanism and small offset faulting where extension depth and geometry before placing the observations occurs predominantly through magma intrusion in the context of global volcanism, rift development, with little crustal thinning [e.g., Ebinger and Casey, geohazard and geothermal potential. 2001; Mackenzie et al., 2005; Maguire et al., 2006]. This narrow zone of localized strain in Quaternary magmatic segments is often termed the Wonji Fault 2. Geodetic Observations and Modeling Belt (WFB). Petrographic modeling, thermodynamic modeling and single clinopyroxene pressure esti- 2.1. Satellite Radar Archive mates have shown that the magmas predominantly [7] InSAR provides a powerful tool to measure fractionate in the upper crust [Rooney et al., 2011]. surface deformation associated with tectonic and Rhyolitic magmatism is associated with regularly magmatic processes across plate boundary zones. – spaced (20 50 km) central volcanoes of diameter The temporal and spatial characteristics of the >10 km, while basalts are usually associated with deformation patterns allow us to discriminate monogenetic vents or fissure eruptions along between dike intrusion, faulting and magma cham- magmatic segments [Mazzarini and Isola, 2010] ber pressure changes [e.g., Wright et al., 2006; (Figure 1). Pritchard and Simons, 2004; Massonnet et al., [4] Between 8 and 9.5°N, the along‐rift peak in 1993]. volume of Quaternary‐Recent volcanism [Abebe [8] Studies of surface deformation are frequently et al., 2007] coincides with a peak in seismic motivated by observations of seismic or eruptive anisotropy of the lithosphere and low seismic activity, in which case the location and timing of the velocities in the mantle [Bastow et al., 2010; Kendall event are already known. However, in this case, no et al., 2006]. Together, these observations suggest external information exists to guide our observations that the greatest melt supply, likelihood of volcanic and we perform a systematic search of all available eruption and geothermal potential lies between satellite radar data, extending back to 1993. The Gedemsa and Dofen volcanoes (Figure 1). South of Envisat background mission (2003–2010) acquired 8°N, geophysical constraints are sparse nonetheless three to four images per year over the MER. Before the distribution of seismicity [Keir et al., 2009], this, the archive is more limited: ERS1/2 collected Moho topography [Tiberi et al., 2005], combined images in 1993, 1997, 2000 and a few images are with geological observations [Ebinger et al., 2000] available form the Japanese Space Agency satellite suggest the deformation is more diffuse, with melt JERS between 1994 and 1996. 2of11 Geochemistry Geophysics 3 BIGGS ET AL.: DEFORMING VOLCANOES IN THE MER 10.1029/2011GC003662 Geosystems G 2.2. Sources of Deformation [10] We identify four areas of significant deforma- tion: (1) Alutu volcano; (2) Corbetti volcano; (3) the area between Bora Berrichio and Tulla Moje vol- cano (hereafter referred to as Bora), and (4) Haledebi volcano in the Angelele segment (Figure 4). At Alutu (1) and Corbetti (2) the circular deforma- tion pattern is centered on the volcano (Figures 1c and 1d). Similar circular deformation at Bora (3) is located in a low‐lying area between the edifices of Bora Berrichio and Tulla Moje (Figure 1b). At Corbetti, four to five fringes of subsidence appear in two independent interferograms (ascending and descending tracks) over 1997–2000. Deformation extends beyond the patch of coherence associated with the edifice so the estimated displacement (14 cm) is a minimum. The JERS interferogram from 1994 to 1996 shows a small amount of uplift (1.4 cm) centered on the volcano but is dominated by a long‐wavelength ramp due to a lack of orbital information. [11] The northernmost signal, Haledebi, is located Figure 1. Main Ethiopian Rift. Insets show example within the Angelele volcanic segment, >10 km south interferograms for (a) uplift at Haledebi (24 August of Hertali volcano. Although the deformation is 2007 to 11 November 2007), (b) uplift at Bora (27 August not associated with a catalogued volcano, a large 2008 to 23 June 2010), (c) uplift at Alutu (17 December volcanic edifice, with fresh‐looking lava flows, 2003 to 18 August 2004), and (d) subsidence at Corbetti is clearly visible in optical satellite images and (23 September
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